The most important factor in creating diverse species is individual and separate environments. Species develop based on their unique environment, which "selects" for particular traits.
Your answer is B.
Sharks belong to a class of fish called "chondrichthyes" (means "cartilage fish") characterized by being jawed fish with skeletons made of cartilage instead of bone.
Sharks and fish with bones (osteichthyes) both have gill slits, scales and paired appendages. So, that would rule out the other answer choices.
Hope that helps!